Electrical heater with temperature cutout

ABSTRACT

A temperature cutout for an electrical heater of the cartridge type having a resistance element wound on an insulating core disposed within a metal sheath and surrounded by insulating material, a first conductor in electrical contact with said resistance element and a second conductor freely disposed within said core, having one end portion normally in electrical contact with said first conductor, said second conductor having a relatively lower coefficient of expansion than said sheath and being sized electrically for self-heating and axial extension and retraction responsive to the current flow, whereby upon attaining a preselected first temperature during heating, contact between said conductors will be broken and said second conductor will cool and contract for rapidly separating said conductors; and upon reaching a relatively lower second temperature during cooling, contact between conductors will be reestablished and said second conductor will heat and extend for rapidly engaging said conductor so that cycling is reduced and arcing is substantially eliminated.

United States Patent [72] Inventor Ronald M. Wrob Sunset Hills, Mo. [21] Appl. No. 878,520 [22] Filed Nov. 20, 1969 [45] Patented Jan. 4, 1972 [73] Assignee Watlow Electric Manufacturing Co.

St. Louis, Mo.

[54] ELECTRICAL HEATER WITH TEMPERATURE CUTOUT 14 Claims, 5 Drawing Figs.

[52] US. Cl 219/335, 2l9/5l2, 219/538, 337/394 [51] Int. Cl. ll05b 1/00 [50] Field ofSearch 219/322, 327, 328, 331, 335, 5l2, 516, 538; 337/141, 393-395, 390, 382; 60/23 56 References Cited UNITED STATES PATENTS 2,829,220 4/1958 Chapman 337/394 3,916,691 1 9 .2 As sa e.

2,373,857 4/1945 Smith ABSTRACT: A temperature cutout for an electrical heater of the cartridge type having a resistance element wound on an insulating core disposed within a metal sheath and surrounded by insulating material, a first conductor in electrical contact with said resistance element and a second conductor freely disposed within said core, having one end portion normally in electrical contact with said first conductor, said second conductor having a relatively lower coefficient of expansion than said sheath and being sized electrically for self-heating and axial extension and retraction responsive to the current flow, whereby upon attaining a preselected first temperature during heating, contact between said conductors will be broken and said second conductor will cool and contract for rapidly separating said conductors; and upon reaching a relatively lower second temperature during cooling, contact between conductors will be reestablished and said second conductor will heat and extend for rapidly engaging said conductor so that cycling is reduced and arcing is substantially eliminated.

. 1 ELECTRICAL HEATER WITH TEMPERATURE CUTOUT BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates in general to electrical heaters, and more particularly, to a temperature cutout therefor.

An object of the present invention is to provide an electrical heater having a temperature cutout with a relatively large temperature differential between the make and break between the conductor contacts thereby reducing cycling normally caused by oscillation between contacts.

An object of the present invention is to provide an electrical heater having an electrical cutout which effects a relatively spontaneous make and break between contacts for substantially eliminating arcing commonly occurring therebetween.

Another object of thepresent invention is to provide an electrical heater having a temperature cutout which assures against heater burnout as the latter will be deenergized automatically upon attainment of a predetermined temperature.

A further object of the present invention is to provide an electrical heater having a temperature cutout which is capable of being immersed within a cleaning or plating solution tank for maximum volume usage as the heretofore necessary floats or other mechanical devices for protection of the heater may be eliminated.

A still further object of the present invention is to provide an electrical heater having a temperature cutout which is compact in construction, inexpensive of manufacture, and reliable and durable in usage.

Another object of the present invention is to provide an electrical heater having a temperature cutout which has a longer life than contemporary heaters.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section taken through an electrical heater constructed in accordance with and embodying the present invention, illustrating same in circuit-closed position.

FIG. 2 is a longitudinal section taken through an electrical heater as shown in FIG. 1, illustrating same in circuit-open position.

FIG. 3 is a transverse section taken on the line 3-3 of FIG. 1.

FIG. 4 is a transverse section taken on the line 4-4 of FIG. I.

FIG. 5 is a transverse section taken on the line 5-5 of FIG. I.

DESCRIPTION OF TI-IEPREFERRED EMBODIMENT Referring now by reference characters to the drawings which illustrate the preferred embodiment of the present invention, A generally designates an electrical heater of the cartridge type comprising a cylindrical sheath 1 preferably constructed of a relatively high expansive metal, such as stainless steel. Sheath l is closed at one end by an end cap 2 and at the other end by a sealing plug 3.

Coaxially disposed within sheath 1 spacedly from cap 2 and plug 3 is a heater core 4 fabricated of insulating material, such as magnesium oxide and having an outside diameter less than the inside diameter of sheath 1 for defining a surrounding space or volume 5, which is filled with a granular, electrically insulating, heat-refractory material 6, such as magnesium oxide or boron nitride. Said heater core 4 is provided with a pair of parallel longitudinal bores 7, 7' extending therethrough, there being milled within opposite ends of core 4 a transverse slot 8, 8' into which said bores 7, 7', respectively, open at their ends. Coiled around core 4 is a resistance element, as formed of conventional metal alloys, such as copper, nickel, chromium and the like; the opposite end portions l0, 10 of which are bent for reception into the related slot 8, 8 and rebent for projection into the proximate end opening of bore 7. Extending into bore 7 through its opposite ends are pins I1, 11, respectively, which engage the end portions 10, I0, respectively, of resistance element 9 for establishing electrical contact therewith. The diameter of each pin 11, II is slightly less than the diameter of bore 7 for developing a snug fit therein.

Endwise of bore 7 pin 11' aligned bore 12 within a spacer heater core 4 and sealing plug 3, within plug 3 for connection of its outwardly extending portion to a source of power. The diameter of spacer core 13 is substantially the same as the inside diameter of sheath I for snug fitting therein.

Said pin 11 extends from the other end of bore 7 through aligned openings l5, 16 in abutting spacer discs 17, I8, respectively, which latter have a diameter substantially the same as the inside diameter of sheath 1. Disc 17 is in engagement against the adjacent end of heater core 4 and disc 18 is in spaced relationship with end cap 2 for cooperating therewith and the intervening portion of sheath 1 to define an end compartment 19 into which pin 11 projects, wherein said pin 1 I is bent back upon itself, as into a hook 20, having its end portion aligned with bore 7' and carrying a diametrically enlarged contact 21 at its end extremity.

Positioned within bores 7' and 12' of heater and spacer cores 4, 13, respectively, and openings 14', I5 and 16' of sealing plug 3 and discs 17, 18, respectively, isa tube 22; the inner end portion of the latter terminates flush with disc 18 opening into compartment 19 and the outer end portion projects beyond sealing plug 3. The outside diameter of tube 22 is substantially the same as the inside diameter of the respective bores 7, 12 and openings 14', l5, 16' for providing a snug fit therein. Received within tube 22 is an elongated conductor 23 having a diameter less than the inside diameter of tube 22 for free movement therein and a length greater than that of tube 22. Conductor 23, at its inner end portion, is provided with a relatively diametrically enlarged contact 24 for engaging contact 21 when extended axially inwardly and for abutting disc 18 when withdrawn outwardly thereby limiting axial movement of conductor 23 within tube 22. Conductor 23 is preferably bimetallic, comprising a rod 25 fabricated of relatively low expansive metal, such as molybdenum or invar, which is fixed at one end to contact 24 and at the other end to a relatively diametrically reduced, medium-to-high expansive, self-heating metal wire 26, such as nichrome. It will be observed that rod 25 is located in the region of heater core 4 and wire 26 is positioned within the zone of the spacer core 13 so that the wire 26 is substantially uneffected by the temperature of the resistance element during operation. Wire 26 is sized electrically for relatively rapid heating and cooling responsive to current flow therethrough, thereby effecting substantially instantaneous axial extension and retraction within tube 22.

Sheath l is diametrically reduced as by swsging, rolling or the like to develop a compact, unitary structure comprised of sheath 1, insulating material 6, cores 4, l3, discs l7, 18, pins 11, 11 and seals 2, 3 with contacts 21 and 24 being in axially aligned relationship. After swaging, conductor 23 is independent of sheath 1 because of its positionment within tube 22. Thus, conductor 23 may beaxially extended and retracted responsive to current supplied thereto because of the selfheating character of wire 26, for effecting circuit-closed and circuit-opened relationship between contacts 21 and 24. For reasons to be further illustrated hereinbelow, the coefficient linear expansion of conductor 23 is relatively less than that of sheath 1 so that the latter will axially expand during heating at a greater rate than conductor 23.

With contacts 2], 24 in circuit-opened position, Heater A is placed within an oven, heat sink, cold sink, or the like, which latter is maintained at a constant predetermined temperature, known in the art as the set" temperature. I'leater A is retained within said oven until the set" temperature is attained whereupon conductor 23 is axially extended through tube 22 for engaging contacts 21, 24. The outer end portion of tube 22 is then crimped or squeezed, as at 27, to maintain conductor 23 in circuit-closed position, after which heater A is removed from the oven and allowed to cool. During cooling,

projects through an axially core 13, interposed between and a registering opening 14 sheath 1 contracts at a relatively greater rate than conductor 23, because of the coefficient of expansion differential for developing a load on conductor 23 thereby enhancing the engagement between contacts 21 and 24.

In operation, heater A is placed within a fluid medium, such as in a tank containing water, gas, or a cleaning or plating solution, and current is supplied to resistance element 9 through conductor 23 and pins 11, 11' respectively. Because of the self-heating, relatively high expansive character of wire 26, the latter will initially extend axially within tube 22 responsive to the current supply for further augmenting the engagement between contacts 21 and 24 and maintaining the load thereon. The axial extension of sheath 1 is determined by the temperature of the resistance element 9 as conducted through insulating material 6 in contrast to wire 26 which is axially extended and retracted responsive to the current flow independent of the temperature of resistance element 9. Thus, wire 26 will extend and retract almost instantaneously where sheath 1 will extend and retract at a relatively slower rate during operation. As the temperature of sheath 1 approaches the set temperature, sheath 1 expands at a relatively greater rate than conductor 23 because of the coefficient of expansion differential therebetween. Because of the initial extension of wire 26 when current is supplied, the temperature at which the contacts 21, 24 are open is relatively greater than the set temperature. With the desired opening temperature known, the set temperature can be readily determined. When the set temperature is exceeded and the opening temperature attained, sheath 1 will have axially expanded relative to conductor 23 to effect disengagement of contacts 21, 24 into circuit-opened position, thereby discontinuing the current supplied to resistance element 9 through conductor 23 and pins 1 1, ll. Wire 26 cools and contracts relatively rapidly with respect to sheath 1 for retracting conductor 23 within tube 22 to effect a substantially instantaneous break and relatively wide gap between contacts 21, 24 for eliminating arcing normally caused by slow break therebetween. Because of the relatively wide gap obtained upon the break between contacts 21, 24, the sheath 1 must, accordingly, cool to approximately the set" temperature for effecting axial contraction sufficient to reengage contacts 21, 24. Thus, the temperature differential between the make and break" conduces to a substantial reduction in cycling normally occurring because of the oscillation between contacts.

Although it is preferred that conductor 23 be bimetallic in construction, it will be understood that conductor 23 may also be constructed of a single metal, such as nichrome, as long as sheath 1 is fabricated of a relatively higher expansive metal. Moreover, it will also be understood that in relatively long heaters more than one heater core 4 with a resistance element 9 wound therearound may be inserted within sheath 1 in series without departing from the essence of the present invention.

Experiments have shown that standard heaters, when tested under circumstances under which the heater is alternatively energized and deenergized, have a life-span in the range of to 40 hours. Heaters A of the present invention, under similar circumstances, have a life-span in excess of 1,500 hours without failure. Thus, it can be seen that by utilizing Heater A of the present invention, arcing will be substantially eliminated thereby promoting a longer life thereto and assuring against heater burnout.

Having thus described my invention, what I claim and desire to obtain by Letters Patent is:

1. An electrical heater comprising a metal sheath, an insulating core disposed within said sheath, first conductor means provided with said core, second conductor means provided with said core, each of said first and second conductor means having an inner terminal and an outer terminal, a source of power, the outer terminal of each conductor means being 'in circuit with said source of power for supplying current through said conductor means, the inner terminal of each conductor means being normally in electrical contact, one of said conductor means having a length sized electrically for axial extension and retraction responsive to curre t flow therethrough, and said sheath and one of said conductor means being diiferentially temperature responsive for effecting disengagement of said inner terminals upon attainment of a predetermined temperature.

2. An electrical device as defined in claim 1 and further characterized by said second conductor means being slidably disposed within said core for relative movement therein responsive to temperature and current flow for effecting substantially instantaneous engagement and disengagement of said inner terminals.

3. An electrical device as defined in claim 2 and further characterized by a tube positioned within said core, said second conductor means received within said tube, said tube having an inner diameter greater than the diameter of said second conductor means for permitting slideable movement therein.

4. An electrical device as defined in claim 3 and further characterized by said second conductor means being bimetallic, having a first portion and a second portion, said first portion being fabricated of a relatively low expansive metal, and said second portion being fabricated of a relatively high expansive metal.

5. An electrical device as defined in claim 4 and further characterized by said first portion being comprised of molybdenum and said second portion being comprised of nichrome.

6. An electrical device as defined in claim 4 and further characterized by said first portion being comprised of invar and said second portion being comprised of nichrome.

7. An electrical device as defined in claim 4 and further characterized by said second conductor means having a coefficient of expansion relatively less than that of said sheath so that as the temperature of the heater increases, the sheath will expand at a greater rate than said second conductor means for effecting disengagement of the inner terminals of said first and second conductor means upon attainment of a predetermined temperature.

8. An electrical device as defined in claim 2 and further characterized by said sheath having a closed end, said insulating core being positioned spacedly from said closed end for defining a compartment, the inner terminals of each first and second conductor means projecting into compartment, said first conductor inner terminal having a hook portion, a contact on said hook portion being axially aligned with said second conductor means.

9. An electrical device as defined in claim 8 and further characterized by said first conductor means comprising a resistance element wound around said core, said resistance element having opposite ends, a first pin constituting said inner terminal within said core engaging one end of said resistance element and projecting into said compartment, a second pin constituting said outer terminal within said core engaging the other resistance element end and projecting endwise of said sheath.

10. An electrical device as defined in claim 4 and further characterized by said second portion having a length and diameter sized electrically for effecting axial extension and retraction responsive to current flow.

11. An electrical device as defined in claim 3 and further characterized by said first conductor means including a resistance element wound around said core; said second conductor means comprising a first portion having a length positioned within said core and a second portion having a length projecting beyond said core; said first portion being axially extended and retracted responsive to the temperature of said resistance element, and said second portion being axially extended and retracted responsive to the current flow therethrough.

12. An electrical device as defined in claim 11 and further characterized by said second conductor means being bimetallic said first portion being fabricated from a relatively low expansive metal, and said second portion being fabricated from a relatively high expansive metal.

13. An electrical device as defined in claim 2 and further characterized by said first conductor means including a resistance element wound around said insulating core, said resistance element having end portions presented at opposite ends of said insulating core, the inner and outer terminals of said first conductor means engaging the related end portions of said resistance element, a spacer core disposed within said sheath and receiving said outer terminals of said first and second conductor means; said second conductor means having a first portion and a second portion, said second portion of said second conductor means constituting said current sensitive portion and being located within said spacer core so as to be unaffected by the sheath temperature.

14. An electrical heater comprising a metal sheath, an insulating core disposed within said sheath, a resistance heater element wound around said insulating core, said resistance heater element having end portions presented at opposite ends of said insulating core, first conductor means provided with said core, second conductor means provided with said core,

each of said first and second conductor means having an inner terminal and an outer terminal, the inner and outer terminals of said first conductor means being in electrical contact with related end portions of said resistance element, a source of power, the outer tenninal of each conductor means being in circuit with said source of power for supplying current through said conductor means, said conductor means being slidably engageable relative to one another, the inner terminal of each conductor means being normally in electrical contact, one of said conductor means having a length including a first current sensitive portion having a greater resistivity and a smaller diameter than a second portion thereof for effecting axial extension and retraction of said conductor means responsive to current flow therethrough, and said sheath and one of said conductor means being differentially temperature responsive for effecting disengagement of said inner terminals upon attainment of a predetennined temperature.

l I I! 

1. An electrical heater comprising a metal sheath, an insulating core disposed within said sheath, first conductor means provided with said core, second conductor means provided with said core, each of said first and second conductor means having an inner terminal and an outer terminal, a source of power, the outer terminal of each conductor means being in circuit with said source of power for supplying current through said conductor means, the inner terminal of each conductor means being normally in electrical contact, one of said conductor means having a length sized electrically for axial extension and retraction responsive to current flow therethrough, and said sheath and one of said conductor means being differentially temperature responsive for effecting disengagement of said inner terminals upon attainment of a predetermined temperature.
 2. An electrical device as defined in claim 1 and further characterized by said second conductor means being slidably disposed within said core for relative movement therein responsive to temperature and current flow for effecting substantially instantaneous engagement and disengagement of said inner terminals.
 3. An electrical device as defined in claim 2 and further characterized by a tube positioned within said core, said second conductor means received within said tube, said tube having an inner diameter greater than the diameter of said second conductor means for permitting slideable movement therein.
 4. An electrical device as defined in claim 3 and further characterized by said second conductor means being bimetallic, having a first portion and a second portion, said first portion being fabricated of a relatively low expansive metal, and said second portion being fabricated of a relatively high expansive metal.
 5. An electrical device as defined in claim 4 and further characterized by said first portion being comprised of molybdenum and said second portion being comprised of nichrome.
 6. An electrical device as defined in claim 4 and further characterized by said first portion being comprised of invar and said second portion being comprised of nichrome.
 7. An electrical device as defined in claim 4 and further characterized by said second conductor means having a coefficient of expansion relatively less than that of said sheath so that as the temperature of the heater increases, the sheath will expand at a greater rate than said second conductor means for effecting disengagement of the inner terminals of said first and second conductor means upon attainment of a predetermined temperature.
 8. An electrical device as defined in claim 2 and further characterized by said sheath having a closed end, said insulating core being positioned spacedly from said closed end for defining a compartment, the inner terminals of each first and second conductor means projecting into compartment, said first conductor inner terminal having a hook portion, a contact on said hook portion being axially aligned with said second conductor means.
 9. An electrical device as defined in claim 8 and further characterized by said first conductor means comprising a resistance element wound around said core, said resistance element having opposite ends, a first pin constituting said inner terminal within said core engaging one end of said resistance element and projecting into said compartment, a second pin constituting said outer terminal within said core engaging the other resistance element end and projecting endwise of said sheath.
 10. An electrical device as defined in claim 4 and further characterized by said second portion having a length and diameter sized electrically for effecting axial extension and retraction responsive to current flow.
 11. An electrical device as defined in claim 3 and further characterized by said first conductor means including a resistance element wound around said core; said second conductor means comprising a first portion having a length positioned within said core and a second portion having a length projecting beyond said core; said first portion being axially extended and retracted responsive to the temperature of said resistance element, and said second portion being axially extended and retracted responsive to the current flow therethrough.
 12. An electrical device as defined in claim 11 and further characterized by said second conductor means being bimetallic said first portion being fabricated from a relatively low expansive metal, and said second portion being fabricated from a relatively high expansive metal.
 13. An electrical device as defined in claim 2 and further characterized by said first conductor means including a resistance element wound around said insulating core, said resistance element having end portions presented at opposite ends of said insulating core, the inner and outer terminals of said first conductor means engaging the related end portions of said resistance element, a spacer core disposed within said sheath and receiving said outer terminals of said first and second conductor means; said second conductor means having a first portion and a second portion, said second portion of said second conductor means constituting said current sensitive portion and being located within said spacer core so as to be unaffected by the sheath temperature.
 14. An electrical heater comprising a metal sheath, an insulating core disposed within said sheath, a resistance heater element wound around said insulating core, said resistance heater element having end portions presented at opposite ends of said insulating core, first conductor means provided with said core, second conductor means provided with said core, each of said first and second conductor means having an inner terminal and an outer terminal, the inner and outer terminals of said first conductor means being in electrical contact with related end portions of said resistance element, a source of power, the outer terminal of each conductor means being in circuit with said source of power for supplying current through said conductor means, said conductor means being slidably engageable relative to one another, the inner terminal of each conductor means being normally in electrical contact, one of said conductor means having a length including a first current sensitive portion having a greater resistivity and a smaller diameter than a second portion thereof for effecting axial extensioN and retraction of said conductor means responsive to current flow therethrough, and said sheath and one of said conductor means being differentially temperature responsive for effecting disengagement of said inner terminals upon attainment of a predetermined temperature. 